If you regularly find yourself looking for open source software (as I do) then you might find this list on Wikipedia (new window) – the free online encylopedia – useful. They maintain a fairly extensive list of open sources packages (new window).
Although, I believe the largest list by a long margin is on SourceForge (new window), although my understanding is that they only list projects which choose to sign up. As an example, OpenOffice.org (new window), a giant open source application, is not listed on SourceForge, to the best of my knowledge.
If you’re wondering what the hell is Open Source software, see this article on Wikipedia… (new window)